1. Field of the Invention
The present invention relates to surgical devices and methods to replace a damaged, diseased, or otherwise painful spinal facet joint.
2. Description of Related Art
Traumatic, inflammatory, metabolic, synovial, neoplastic and degenerative disorders of the spine can produce debilitating pain that can have severe socioeconomic and psychological effects. One of the most common surgical interventions today is arthrodesis, or spine fusion, of one or more motion segments, with approximately 300,000 procedures performed annually in the United States. Clinical success varies considerably, depending upon technique and indications, and consideration must be given to the concomitant risks and complications. For example, it has been shown that spine fusion decreases function by limiting the range of motion for patients in flexion, extension, rotation, and lateral bending. Furthermore, it has been shown that spine fusion creates increased stresses and, therefore, accelerated degeneration of adjacent non-fused motion segments. Additionally, pseudoarthrosis, as a result of an incomplete or ineffective fusion, may reduce or even eliminate pain relief for the patient. Also, the fusion device, whether artificial or biological, may migrate out of the fusion site.
Recently, several attempts have been made to recreate the natural biomechanics of the spine by use of an artificial disc. Artificial discs provide for articulation between vertebral bodies to recreate the full range of motion allowed by the elastic properties of the natural intervertebral disc which directly connects two opposed vertebral bodies.
However, the artificial discs proposed to date do not fully address the mechanics of motion of the spinal column. In addition to the intervertebral disc, posterior elements called the facet joints help to support axial, torsional and shear loads that act on the spinal column. Furthermore, the facet joints are diarthroidal joints that provide both sliding articulation and load transmission features. The effects of their absence as a result of facetectomy has been observed to produce significant decreases in the stiffness of the spinal column in all planes of motion: flexion and extension, lateral bending, and rotation. Furthermore, contraindications for artificial discs include arthritic facet joints, absent facet joints, severe facet joint tropism or otherwise deformed facet joints.
U.S. Pat. No. Re. 36,758 to Fitz discloses an artificial facet joint where the inferior facet, the mating superior facet, or both, are covered with a cap. The cap requires no preparation of the bone or articular surfaces; it covers and, therefore, preserves the bony and articular structure.
The capping of the facet has several potential disadvantages. If the facet joint is osteoarthritic, a cap will not remove the source of the pain. Additionally, at least in the case of surface replacements for osteoarthritis femoral heads, the capping of articular bone ends has proven to lead to clinical failure by means of mechanical loosening. The clinical failure is hypothesized to be a sequela of disrupting the periosteum and ligamentum teres femoris, both serving a nutrition delivery role to the. femoral head, thereby leading to avascular necrosis of the bony support structure for the surface replacement. Another potential disadvantage is that in order to accommodate the wide variability in anatomical morphology of the facets, not only between individuals but also between levels within the spinal column, a very wide variety of sizes and shapes would be required.
U.S. Pat. No. 6,132,464 to Martin discloses a spinal facet joint prosthesis that is supported on the lamina, which is sometimes also referred to as the posterior arch of the vertebra. Extending from this prosthetic support structure are inferior and/or superior blades that replace the cartilage at the facet joint. Like the Fitz design, the Martin prosthesis generally preserves existing bony structures and therefore does not address pathologies which affect the bone of the facets in addition to affecting the associated cartilage. Furthermore, the Martin invention requires a mating condition between the prosthesis and the lamina, or the posterior arch, that is a thin base of curved bone that carries all four facets and the spinous process. Since the posterior arch is a very complex and highly variable anatomic surface, it is very difficult to design a prosthesis that provides reproducible positioning to correctly locate the cartilage-replacing blades for the facet joints.
Another approach to surgical intervention for spinal facets is provided in WO9848717A1 to Villaret. While Villaret teaches the replacement of spinal facets, the replacement is interlocked in a manner to immobilize the joint.
Facet joint replacement in conjunction with artificial disc replacements represent a novel solution to recreating a fully functional motion segment that is compromised due to disease or trauma. Together, facet joint and disc replacement can eliminate substantially all sources of pain, return full function and range of motion, and restore the natural biomechanics of the spinal column. Additionally, degenerative or traumatized facet joints may be replaced in the absence of disc replacement when the natural intervertebral disc is unaffected by the disease or trauma.
It would therefore be an improvement in the art to provide a device and a method for the replacement of multiple vertebral facets and a portion of their associated bony structure so as to remove the source of traumatic, arthritic, or other disease related pain.
It is an object of the invention to provide a multiple vertebral facet replacement prosthesis for a diseased or traumatized vertebra with painful or compromised facet joints.
It is another object of the invention to provide a method of replacing multiple vertebral facets for a diseased or traumatized vertebra with painful or compromised facet joints.
It is yet another object of the present invention to provide a kit of multiple facet prostheses for the convenient replacement of one or more levels of facet joints.
In one preferred embodiment, the inferior bilateral facets of a superior vertebra are resected at the base of the facets where they connect to the posterior arch. Flanges are oriented relative to the body of the prosthesis such that when the flanges are placed against the pedicles and in a manner such that the planar surfaces of the flanges are perpendicular to the respective axes of the pedicle""s interior bone canal, the articulating surfaces of the inferior bilateral facet prosthesis will be properly positioned to replicate the articulating surfaces of the natural facets. Each flange includes a hole for the passage of a fastener to securely attach the prosthesis to the pedicle. The fastener can be a screw, spike, tack, staple, modular post, or the like.
Alternatively, or in addition, the superior facets of an inferior vertebra are resected at the base of the facets where they connect to the pedicles. As with the bilateral inferior facet prosthesis, a bilateral superior facet prosthesis is attached to the inferior vertebra by means-of fasteners which engage holes on mounting flanges and the pedicles.
The fasteners or the bone contacting surfaces of the flanges may be porous coated to promote bone ingrowth in order to achieve long term fixation. The porous coating may carry osteoconductive agents, such as hydroxylapatite, calcium sulfate, or demineralized bone matrix. Alternatively, the porous coating may carry osteoinductive agents, such as bone morphogenic proteins, including rhBMP-2 and rhBMP-7.
Alternative embodiments of the present invention include the replacement of different combinations of facets on a single vertebra. One embodiment provides for the replacement of the left inferior and left superior facets by a single prosthesis. Another embodiment provides for the concurrent replacement of all four facets by a single prosthesis. The four facet prosthesis can be used in conjunction with a superior bilateral facet replacement and an inferior bilateral facet replacement to provide a two level replacement of facet joints. Alternatively, two of the four facet prostheses can be used in conjunction with a superior bilateral facet replacement and an inferior bilateral facet replacement to provide a three level replacement of facet joints, where the middle level facet replacement is accomplished by the two adjacent four facet prostheses.
The present invention has numerous advantages over the prior art. Among other things, the present invention provides for the convenient, simultaneous replacement of multiple combinations of facets on a single vertebra: two superior facets, two posterior facets, two left facets, two right facets, or all four facets. Furthermore, multiple levels of facet joints can be replaced by selecting from a kit of multiple facet joint prostheses. Another advantage of the present invention is a precise and tight attachment of the prosthesis to bones, as opposed to prosthetic surfaces that rely on mating with highly complex and variable external surfaces of the vertebra, such as the posterior arch. Another advantage is that the optional porous coating is placed into interior bone spaces where porous coatings have proven to achieve bone ingrowth for excellent long term fixation strength. This ability to achieve bone ingrowth is uncertain for the prior art devices that engage the external bone surfaces of the vertebra. Yet another advantage lies in the removal of the facet bone structure. More particularly, where the facet bone is involved in the disease pathology or the trauma that compromised the articular or cartilaginous surface of the facet, resection provides a means for ensuring that all pain associated with the disease or trauma is removed. With prior art devices, the bony structure of the facets is generally left intact.
The above, and other, objects, features and advantages of the present invention will become apparent from the following description which is to be read in conjunction with the accompanying drawings.